Advances in fiber optic technology have stimulated interest in fiber optic chemical sensors (FOCS). In such devices, the presence of a chemical analyte induces a chemical reaction with a reagent. The reaction is detected as a change in an optical signal. In well known fluorescent methods the change in optical signal may be due to fluorescent material resulting from or being stimulated by the chemical reaction.
FOCS, in general, offer the well known advantages of optical fibers, i.e., small size and freedom from electromagnetic interference. For medical applications they offer the advantage of stability and potential for compatibility with a medical environment, either in a laboratory or "in vivo".
Although fluorescence FOCS has been demonstrated for a number of analytes, they exhibit a limited lifetime in certain environments. Dyes, for example, which are often used as sensors are susceptible to photochemical degradation. Additionally, fluorescence sensors require the use of relatively complex signal detection equipment capable of distinguishing a fluorescence signal induced by an analyte from background fluorescence. Concern about background fluorescence limits the use of plastic fibers which may be safer and more economical in many applications such as medical applications. Further, it is difficult to find a reaction for each possible analyte which leads to a fluorescent product, since only about ten percent of all molecules fluoresce.
A paper "An Intravascular Protein Osmometer", J. Hansen and R. Brace, Am. Journal of Physiology, 244 (5) pp H726-729 (1983), describes a device for measuring the concentration of protein in circulating blood, using osmotic pressure. In the Hansen and Brace device, two identical capillary tubes are placed in a sample. One of the tubes is open to sample pressure, while the other is terminated with an osmotic membrane, formed by a dialysis fiber. Osmotic pressure is determined by the difference in pressure in the two capillary tubes. A significant disadvantage of the Hansen and Brace device was that several minutes were required for the pressure to reach equilibrium. The equilibration time could be reduced by increasing the area of the osmotic membrane, but about half of the response time was attributed to the equilibration time within a capillary.
Although osmotic pressure has been used to measure concentrations of analytes of biological and medical interest, well-known methods of measuring osmotic pressure, using a device generally termed an osmometer are, in general, not satisfactory for this purpose, primarily because of shortcomings in common osmometric methods. For example, a review of osmometry by Schaeffer, "Modern Aspects of Membrane Osmometer Design", Colloid and Polymer Science, 257, pp 1007-1108, (1979) identifies the desirability of improvements in osmometric methods such as shorter response time, eliminating a free boundary between a solvent and a solution, a capability to externally modify pressure difference and temperature compensation. Such improvements are indeed required if osmotic pressure measurement is to be commercially useful as a method of chemical detection.